Automotive sensors for angular measurement of camshafts and crankshafts are exposed to harsh environments (temperature variations between −40 and 150° C., mechanical vibrations, oil contamination, etc). Magnetic field sensors are often preferred to other sensor types due to their robustness and low production costs. In general, there are two widely used magnetic sensing arrangements that include a patterned wheel mounted on a rotating shaft and a magnetic field sensor.
In one arrangement, the patterned wheel is magnetically coded around its edge. The alternating magnetic regions pass the sensor and induce a magnetic field. If the pole wheel rotates, the normal component of the induced field at the position of the sensor has a sinusoidal-like shape. Pole wheels with different magnetic patterns are used for a variety of applications. Speed of a shaft, such as in an anti-lock brake system (ABS), can be obtained by using a regular patterned wheel, but for angular position measurements, such as for a camshaft, crankshaft, etc, an irregular pattern is required.
Another arrangement uses a toothed wheel which is coded by the length of the teeth and tooth spaces. A permanent magnet (back-bias magnet) placed behind the sensor creates a constant magnetic field which is influenced by the rotating wheel. If a tooth is in front of the magnet, the field at the sensors position is high and if a tooth space is in front of the magnet, the field is low. The rotating toothed wheel shapes the magnetic field at the position of the sensor element resulting in a field which has similar shape as described on pole wheels. However, an additional DC component—which is caused by the unipolar nature of the magnetic field—appears and therefore the signal has no zero value. Regular patterned wheels are used to transmit mechanical forces.
In sensing arrangements such as these, the magnetic field sensor element (e.g. Hall, Giant Magneto Resistance, GMR, etc.) converts the applied magnetic field into a linear proportional electrical signal. Signal processing is used to convert the sinusoidal-like shaped signal into a binary sequence which is a rough representation of the pattern on the wheel. Knowing the pattern, the rotational speed and angular position can be determined from this binary signal.
Packaging and mounting tolerances, mechanical vibrations and temperature variations cause variations of the air gap between the patterned wheel (pole wheel or toothed wheel) and the sensor element. Therefore, the magnetic field at the sensor's position also can vary, resulting in variations of the electrical signal shape and a displacement of the peak and zero value positions in the signal. Known signal processing concepts do not take into account the gap dependent waveform variations and displacement of peak and zero value positions, which can result in angle measurement errors.
For instance, signal processing is used to convert the analog output voltage of the sensor element into angular position information. Processing power on single chip integrated sensors is limited and therefore simple circuits or algorithms are used for conversion.
Many sensor concepts remove the DC component of the analog output voltage of the differential sensor voltage v(x). This makes the subsequent signal processing applicable on both sensing arrangements using pole wheels or toothed wheels. Therefore, removing of the DC component is also common on pole wheel measurements.
A frequently used (analog) solution to remove the DC component is to estimate and remove the average value of the analog signal. Other (digital) solutions calculate the mean value of the last maxima and minima, which leads to comparable results. Yet another solution is to adjust the DC component until a 50% duty cycle of the binary output signal is reached.
Differential measurement is also a well known strategy to remove the DC component. Two sensor elements measure the magnetic field at different positions around the edge of the wheel. The DC component of the analog output voltage of both sensor elements is similar and the AC component is shifted by their separation distance. Subtracting these two signals minimizes the DC component and doubles the AC amplitude if the separation distance equals 1 MR. Due to the imbalances between the two sensor elements, parasitic offset voltages, etc., only about 90 to 95% of the DC component can be removed using differential measurement with Hall elements. Therefore the above described strategies are additionally used to remove the (small) remaining DC component. After removing the DC component, the waveform obtained from measurements using toothed wheels or pole wheels are similar.
Zero crossing detection is used to convert the analog sensor signal in a binary signal, which is an electric image of the pattern on the wheel (toothed wheel or pole wheel). Knowing this pattern, angle (and speed) information can be determined by evaluating the binary signal. Sensor solutions (using Hall elements) are able to detect zero crossings with high accuracy and the 1σ-jitter is below 0.001 MR. However, due to gap variations displacement of the zero value (up to 0.28 MR) can appear.
Displacements resulting from gap variations typically cannot be avoided even if different sensing arrangements are used. Thus, that DC cancellation is an insufficient strategy to obtain highest angular accuracy and that the displacement must be taken into account. A shown in U.S. Pat. No. 7,208,944 (incorporated by reference herein), digital signal processing can be used to compensate the displacements caused by air gap variations. With this signal processing also other effects (variations of the magnetization strength of the pole wheel etc.) can be compensated.
To allow digital signal processing, a clock signal is required. To reduce complexity of signal processing the frequency of the clock signal must be synchronous to the revolution speed of the engine. The revolution speed of an engine varies typically between 0 and 14000 rpm and a Phase Locked Loop (PLL) can be used to follow this speed variation and generate a clock signal for the digital signal processing unit. However, due to the impulsive forces acting onto the pistons during explosion and compression stroke the revolution speed of the crankshaft varies slightly as shown in FIG. 21. A conventional PLL cannot follow these frequency variations and phase differences between the revolution speed (pattern frequency on the pole wheel) and the PLL frequency appear.
For these and other reasons, there is a need for the present invention.